加里东期上扬子区前陆盆地演化过程中的层序特征与地层划分

层序地层的主控因素包括全球海平面变化、构造沉降、气候、物源供给等方面 ,在不同性质的盆地内它们所起的作用大小不一 ,在前陆盆地中以构造沉降控制为主。上扬子区在加里东运动过程中形成前陆盆地 ,主要经历了三次挤压、挠曲沉降至松驰、抬升过程。一是中奥陶世至晚奥陶世临湘期 ,二是晚奥陶世五峰期至早志留世龙马溪期 ,三是早志留世石牛栏期至中志留世。在盆地演化过程中 ,由于前陆隆起露出水面 ,使物源供给方向和性质改变 ,造成隆后盆地沉积环境由开放变为半闭塞 ,由碳酸盐岩转变为碎屑岩沉积。以此重大的转换为界 ,将上扬子区的中奥陶统至中志留统划分为 2个二级层序 ,奥陶系—志留系界线放在第二个二级层序的最大海泛面之顶 ,也就是观音桥组的顶 ,此结果与岩石地层清理成果相一致。以海平面相对变化和沉积物的转变进一步划分成 4个三级层序 ,又根据三级层序的体系域的演化相应地划分出岩性组。因此将临湘组与宝塔组归并成宝塔组 ,体现了海水逐渐变浅、向上加积的过程 ,且它们间为一渐变过渡 ,在碳酸盐浅滩和盆地边缘之处 ,两者划分不开 ,为了便于填图与对比 ,将其划归在一起。由于观音桥组分布不稳定 ,五峰组与龙马溪组下部岩性相似 ,为了油气的评价和填图的可操作性 ,将五峰组、观音桥组、龙马?

CHARACTERISTIC OF SEQENCE AND STRATIGRAPHICAL DIVISION IN EVOLUTION OF UPPER YANGTZE REGION DURING CALEDONIAN

The sequence stratigraphy is mainly controlled by the eustatic fluctuation, tectonic subsidence, climate and provenance. The tectonic subsidence is the main controlling factor in the foreland basin, which went through three periods, namely press, flexure subsidence to laxitas, uplift. The first period is from Middle Ordovician to Linxiang Stage, the second from Wufeng to Longma Stages, the third is Shiniulan Stage to Middle Silurian. Two second order sequences and four third order sequence are recognizable from Middle Ordovician to Middle Silurian. The first second order sequence is from the Dawan to Baota Stages. The top boundray is type Ⅰ.The second second order sequence is from Wufeng to Samao and its top is also type Ⅰ. The mfs of the top of the second second order sequence contains the Ordovician Silurian Boundary, i.e., the so called Guanyinqiao Bed. For mapping and petroleum evaluation, the strata of the upper Yangtze are divided again. The Linxiang and Baota Formations have merged into Baota Formation, The Wufeng, Guanyinqiao and lower Longmaxi Formations are amalgamated into Longmaxi Formation. The upper Longmaxi Formation is renamed Xintan Formation.